Multigroup energy representation for fast Monte Carlo estimation of radioisotope yields from electron-beam spectra

Abstract

Monte Carlo (MC) simulations are widely used to estimate radioisotope production through nuclear reactions. However, achieving high accuracy typically requires simulating more than 10^8 particles, which is computationally demanding, particularly when many simulations are required, such as in optimization loops. In a previous study, we demonstrated that molybdenum-99 yields from bremsstrahlung-driven photonuclear reactions induced by laser-accelerated electron beams can be estimated up to four orders of magnitude faster by employing a multigroup energy representation, in which a limited set of MC precomputed group yields is used to reconstruct the total yield for an arbitrary source spectrum. This work expands this approach by showing that the multigroup representation also provides a deeper understanding of the relative contribution of different electron energies to the final isotope yield. Applying the method to 640 different spectra revealed how the choice of group structure—such as the number of groups and their energy widths—affects both accuracy and efficiency. The results show that the proposed approach reproduces full MC yield estimates with median errors below 5%, reaching values as low as 0.3%.